Dewhurst will have plenty of money to fight back against attacks from the right - he has two super PACs on his side spending millions on his behalf. Bruning never had such a luxury.
Cruz, on the other hand, is running as an unadulterated conservative, making his ideological message the hallmark of his campaign. He's excited conservative activists and become the darling of national conservative figures like Sen. Jim DeMint, who has aggressively challenged establishment favorites.
If Dewhurst loses to Cruz in a runoff, that would make three straight Senate primaries where Republicans with close ties to the establishment end up losing to insurgents. It would be very hard to simply dismiss a Cruz victory as being caused by outside groups or by Dewhurst's campaign blunders. Something bigger is happening.